The present invention relates to computer controlled extrusion of a liquified material. One useful application for computer-controlled extrusion techniques is in rapid prototyping of models or objects. In particular, the present invention is an extrusion apparatus from which a liquified material is extruded at a variable extruded pump flow rate that dictates the velocity profile of the extrusion apparatus.
A rapid prototyping system involves the making of three-dimensional objects based upon design data provided from a computer aided design (CAD) system. Examples of apparatus and methods for rapid prototyping of three-dimensional objects by depositing layers of solidifying material are described in Crump U.S. Pat. No. 5,121,329, Batchelder et al. U.S. Pat. No. 5,303,141, Crump U.S. Pat. No. 5,340,433, Batchelder U.S. Pat. No. 5,402,351, Batchelder U.S. Pat. No. 5,426,722, Crump et al. U.S. Pat. No. 5,503,785, and Abrams et al. U.S. Pat. No. 5,587,913, all of which are assigned to Stratasys, Inc.
A typical rapid prototyping system of the prior art has an extrusion head for extruding a bead of a liquified material, an extrusion head motor for driving the extrusion head, an extrusion pump for providing the material to the extrusion head, and a control for providing control signals to both the extrusion head motor and the extrusion pump. The control signals control motion and velocity of the extrusion head and operation of the extrusion pump.
The extrusion head is driven at a constant head velocity along a path defined by a series of vertices. This head velocity is preselected so as to accomplish the general goal of causing the extrusion head to quickly navigate the path while minimizing the extrusion head's displacement from the path. As a result, the head velocity is preselected to equal a sufficiently low value so that when the extrusion head proceeds through any of the vertices where its heading is caused to change greatly, the deviation by the extrusion head from the path, or following error, will be contained within an allowable error allowance.
The liquified material delivered by the extrusion head has a bead of a cross-sectional area that should ideally be controlled to create an accurate prototype. This bead size is related to both the extruded pump flow rate, which is the rate at which material is extruded from the extrusion head, and to the extrusion head velocity. The bead size will increase if the head velocity is held constant while the extruded pump flow rate is increased. Conversely, the bead size will decrease if the extruded pump flow rate is held constant and the head velocity increased. In the past, an unvarying bead width has generally been desired; therefore, both the extruded pump flow rate and the extrusion head velocity have remained constant as well.
While this method of holding both the head velocity and the extruded pump flow rate constant allows for an unvarying bead width of material to be extruded while keeping any displacement of the extrusion head within the allowable error allowance, there are several problems with this method.
First, the extrusion head velocity is held at a constant value substantially less than the maximum velocity at which the extrusion head can be driven, thereby requiring a longer amount of time to create a model than might otherwise be possible if the extrusion head were driven at a greater velocity. This is particularly problematic in view of the growing demand for the creation of larger and more complex models.
Second, the extrusion head velocity cannot be slowed at any vertices that might benefit from a slower head velocity. Although the displacement of the extrusion head from the path is contained within the allowable error allowance, the amount of displacement could be further reduced if the extrusion head velocity were reduced at these vertices. In addition, the tolerable error allowance could be reduced if head velocity varied depending upon the characteristics of the path along which the extrusion head is driven.
There is therefore a need for an extrusion system that can achieve a greater throughput and a better product by varying the head velocity of the extrusion head and causing the extrusion head to more quickly navigate the predefined path.